1. Field
This application relates to elastomeric formulations suitable for use in thin films and sheets.
2. Description of the Related Art
Elastomeric materials have long been used in a variety of fields, including such fields as diapers. In the field of diapers, elastomeric materials have been used in several parts of the diaper, including both elastic cuffs around the waist and legs as well as the adhesive tabs used to fasten the diaper around the torso of a child. Both the body and tab portions of the diaper utilize elastomeric films. Films in elastic portions of products such as diapers may be exposed to extended periods of mechanical stress as well as elevated temperatures due to their close proximity to the body.
These films may include several primary components. The first component is typically a rubber or rubber-like block copolymer, such as those sold by Kuraray America, Inc. (Pasadena, Tex.) as Septon® 4000 series polymers. Septon polymers are S-E-EP-S polymers (styrene-ethylene-ethylene/propylene-styrene) that are hydrogenated styrenic block co-polymers with a mid block derived from a mixture of isoprene and butadiene. The second component is oil which is a plasticizer that helps make the composition softer. A third component such as polypropylene, polystyrene, and other polymers may also be included. Similarly, a wide variety of other additives may also be included in such compositions.
U.S. Publication No. 2009/0018253 discloses thermoplastic elastomer compositions which blend thermoplastic elastomers with certain performance additives. The '253 publication particularly discloses the use of styrenic block copolymers such as styrene-ethylene-butylene-styrene (SEBS), styrene-butadiene-styrene (SBS), styrene-isoprene-styrene (SIS), and styrene-ethylene-propylene-styrene (SEPS); and its blends with polyolefins (especially polypropylene), ethylene propylene dienemonomer (EPDM) rubber, and blends of polyolefins and EPDM rubber. The '253 publication also discloses performance additives, including both aliphatic and aromatic additives. These compositions were in many cases also mixed with oil and reportedly provided improved mechanical properties and processability. However, the '253 publication does not describe particular applications such as films, does not describe the use of SEEPS polymers, and does not does not describe the use of vinylarene resins (such as polystyrene) in combination with the block copolymers and additives.
In another known composition disclosed in U.S. Pat. No. 6,410,129, elastomeric films are disclosed which are made of three primary elements. These include Septon® 4033, oil, and polystyrene. In one exemplary embodiment of U.S. Pat. No. 6,410,129, the composition of a thin film includes 55% Septon 4033®, 15% polystyrene PS210, and 30% Drakeol® mineral oil. However, the elastomeric materials used to make these films are relatively difficult to process, may delaminate from adjacent layers when used in products such as diapers, and may gradually tear under stress over time during the life of a product such as diapers. The ability to resist such tearing is a property referred to as “slow tear resistance.”